The present invention describes polymer electrolyte membranes. More specifically, the present invention describes a polymer electrolyte membrane with special properties that are optimized for use in direct methanol fuel cells.
The use of gasoline-powered internal combustion engines has created several exhaust gas-related environmental problems. Pollution control measures cannot completely cleanse the environment of exhaust gases that are produced upon burning of non-renewable fossil fuel.
Tremendous efforts have been directed towards finding a satisfactory solution to the pollution problems that currently plague the environment. One possible solution is the use of fuel cells. Fuel cells are highly efficient electrochemical energy conversion devices which directly convert the chemical energy derived from a renewable fuel into electrical energy. Significant research activity has focussed on the development of proton-exchange membrane fuel cells, which have shown promise in low-temperature portable applications. The proton-exchange membrane fuel cells have a polymer electrolyte membrane disposed between a positive electrode (cathode) and a negative electrode (anode). The polymer electrolyte membrane is composed of an ion-exchange polymer (i.e. ionomer) and its role is to provide a means for ionic transport and prevent mixing of the molecular forms of the fuel and the oxidant.
Polymer electrolyte membranes intended for H2/O2 fuel cell applications were initially made by condensation of phenolsulfonic acid and formaldehyde. These membranes had certain structural limitations and were seen to be brittle, prone to cracking when dried, and subject to rapid hydrolysis.
Amberplex C-1 and Amberplex A-1 membranes (Rohm and Haas Company), described in U.S. Pat. No. 2,913,511, were later suggested for use in H2/O2 fuel cells.
Polymer electrolyte membranes based on partially sulfonated polystyrene have also been investigated for use in H2/O2 fuel cells.
The ionomer has typically been blended with inert polymers such as fluorinated polymers in order to improve the oxidative, thermal and dimensional properties of polymer electrolyte membranes. However, introduction of an inert matrix does not alter the chemical and thermal properties of the ionomer. Moreover, large proportions of the inert matrix may increase the ionic resistance of the polymer electrolyte membrane. This reasoning led to the development of xe2x80x9cD-Membranes,xe2x80x9d which were fabricated by cross-linking styrene-divinylbenzene with an inert fluorocarbon matrix, followed by sulfonation. Appleby and Jaeger, Energy, 1986, 11, 137. However, the lifetimes of these H2/O2 fuel cells were less than optimal due to degradation resulting from attack on the weak xcex1-Cxe2x80x94H bond in the polymer structure.
In order to address the stability problem observed with the xe2x80x9cD-Membranes,xe2x80x9d General Electric Company developed the xe2x80x9cS-Seriesxe2x80x9d of membranes which were fabricated from homopolymers of xcex1,xcex2,xcex2-trifluorostyrene-sulfonic acid. Chapman, Proc. 7th Intersoc. Energy Conv. Eng. Conf., 1972, 466; and Hodgdon et al., U.S. Pat. No. 3,341,366, issued Sep. 12, 1967. Although these membranes exhibited good chemical and thermal properties, their physical properties were less than adequate.
Soon after, DuPont Chemical Company developed the NAFION(trademark) series of polymer electrolyte membranes which found use in fuel cell space applications. Nafion has been considered as the membrane of choice. However, high cost associated with the use of Nafion membranes is a disadvantage.
A liquid-feed fuel cell such as a direct methanol fuel cell (DMFC) has shown promise. A DMFC uses an aqueous methanol solution at temperatures as low as 60-90xc2x0 C. Current state-of-the-art liquid-feed fuel cells have a carbon-supported Pt-Ru catalyst at the anode, carbon-supported Pt catalyst at the cathode and a polymer electrolyte membrane positioned between the anode and the cathode. An aqueous solution of an organic fuel is circulated at the anode by a pump element, which can be a conventional pump, or a more inactive pump such as vapor lock or the like. Oxygen or compressed air is supplied at the cathode. An ideal polymer electrolyte membrane should be impermeable to the organic fuel. However, the membrane may allow permeation of some organic fuel from the anode to the cathode. This is termed xe2x80x9cfuel crossover.xe2x80x9d Fuel crossover decreases fuel efficiency and fuel cell efficiency.
The most advanced DMFC systems use a Nafion 117 perfluorocarbon proton-exchange membrane (DuPont Chemical Company) as their electrolyte. Nafion 117 membranes demonstrate high conductivity and possess high power and energy density capabilities. However, use of Nafion 117 membranes in DMFCs is associated with disadvantages including very high cost, and a high rate of methanol permeation from the anode compartment, across the polymer electrolyte membrane (i.e. Nagion 117 membrane), to the cathode. This xe2x80x9cmethanol crossoverxe2x80x9d lowers the fuel cell efficiency. The diffusion coefficient for methanol in Nagion 117 membranes has been reported to be in the order of 10xe2x88x925 cm/s. Verbrugge, J. Electrochem. Soc., 1989, 136, 417.
Methanol crossover results in decreased fuel cell voltage and efficiency due to the oxidation of methanol to carbon dioxide at the cathode. Therefore, it is important that the extent of methanol crossover be as small as possible. Consequently, research efforts have focussed on methods of decreasing methanol crossover in DMFCs so that higher cell voltage and efficiency may be achieved.
One approach aimed at decreasing the methanol crossover rate involved polymer-bonded particle hydrates based on tin-mordinite. Lundsgaad et al., Proc. Electrochem. Soc., 1993, 140, 1981; and Lundsgaad et al., Solid State Ionics, 1994, 72, 334. These membranes exhibited lower methanol diffusion, however, they showed modest conductivities at room temperature and decreased solvent uptake.
Another approach aimed at decreasing methanol permeation across the electrolyte membrane involved three-layered electrolyte systems based upon the use of metal hydride films serving as methanol-impermeable proton conductors sandwiched between proton-permeable electrolyte membranes. Pu et al., J. Electrochem. Soc., 1995, 142, L119. Such systems have been reported to have low methanol permeability when operated in H2/O2 fuel cells. However, no data is available for their application in aqueous liquid-feed DMFCs.
Polymer electrolyte membranes comprising polybenzimidazole films doped with phosphoric acid have also been investigated for use in DMFCs. Wainwright et al., J. Electrochem. Soc., 1995, 142, L121. Although these membranes have demonstrated decreased methanol permeability in vapor-feed fuel cells, they have not been amenable for use in liquid-feed DMFCs as they only display adequate conductivities at temperatures as high as 150-200xc2x0 C.
Several polymer electrolyte membranes have been fabricated to decrease methanol crossover and enhance fuel cell efficiency. However, these membranes have significant limitations when applied to low-temperature liquid-feed DMFCS. Consequently, there is a need for polymer electrolyte membranes which are functional in low-temperature liquid-feed DMFCS, and that display low methanol crossover rates and high fuel cell efficiencies.
The present invention provides a novel polymer electrolyte membrane composed of sulfonated polystyrene cross-linked with divinylbenzene, referred to herein as PSSA polystyrene sulfonic acid and poly(vinylidene fluoride) xe2x80x9cPVDFxe2x80x9d. The material used could also be sulfonated and cross-linked polystyrene/divinylbenzene and PVDF. Both materials can be called PSSA-PVDF. A preferred mode uses these materials in a fuel cell. In one preferred embodiment of the present invention the fuel cell is a liquid-feed fuel cell. In another preferred embodiment the fuel cell is a direct methanol fuel cell.
The present invention also provides a novel fuel cell comprising a polymer electrolyte membrane composed of polystyrene sulfonic acid and poly(vinylidene fluoride). In a preferred embodiment said fuel cell is a direct methanol fuel cell.
The present invention further provides a method of decreasing methanol crossover rates in a direct methanol fuel cell. Methanol crossover rate in said direct methanol fuel cell is decreased by using a polymer electrolyte membrane which is composed of polystyrene sulfonic acid and poly(vinylidene fluoride).
The present invention also provides a method of enhancing efficiency of a direct methanol fuel cell. The efficiency of said direct methanol fuel cell is enhanced by using a polymer electrolyte membrane which is composed of polystyrene sulfonic acid and poly(vinylidene fluoride).
The present invention additionally provides a method of enhancing electrical performance of a direct methanol fuel cell by using low flow rates of oxygen at the cathode of the fuel cell.